Work over the past sixteen years has reclassified Blastocystis hominis as a protozoan, removing it from the yeasts where it had languished for seventy years. B. hominis is now firmly established as a member of the Sporozoa, an intestinal parasite and pathogen of man and primates. When B. hominis is present in large numbers in stool contents, marked diarrhea is usually experienced. The drug of choice is metronidazole. Patients with severe infection have been treated at the Clinical Center. There was one fatal case.